This invention relates to rotors for axial flow turbines, such as gas turbines. In particular, this invention relates to such rotors wherein ceramic rotor blades are mounted to a metallic rotor disk.
Gas turbine efficiency is directly related to the temperature of the turbine working gas. Ceramic turbine blades, which can withstand exposure to very high temperature gases, are advantageously used to provide improved turbine efficiency. The ceramic turbine blades can be mounted to a metallic disk by providing an enlarged base portion on each blade which engages an undercut groove on the metallic disk. The direct mounting of the ceramic to the metal disk can cause excess pressure and stresses on the supporting surface of the ceramic. In prior art designs, such as shown in published German Patent Application No. 2,108,176, intermediate layers of cushioning material are placed between the ceramic base portion and the undercut supporting surface of the disk to prevent stress and breakage of the ceramic base portion. In this prior art design, a highly elastic metal felt, capable of relatively large deformation is used. This cushion pad permits substantial movement of the turbine blade under centrifugal forces, from its position when the rotor is stationary, to a different position relative to the disk when the rotor is at full speed. In this prior design, the entire supporting surfaces of the base portion are always in contact with the cushioning pad. In the event of dimensional changes in the blade base or rotor because of manufacturing tolerances, the blade base may be subjected to additional stresses or bending moments on account of this mounting technique. In some instances, the stresses generated may exceed the centrifugal forces on the base and cause fracture of the ceramic.
It is therefore an object of the present invention to provide an axial flow turbine rotor with an improved arrangement for the mounting of ceramic rotor blades to a metallic rotor disk.
It is a further object of the invention to provide such an arrangement wherein bending moments on the base of the ceramic rotor blade do not result in excess stress in the ceramic.